1. Field of the Invention
The present invention relates to a reception system and a digital broadcasting system.
2. Related Art of the Invention
As the development of digital video and audio technology using MPEG2, a digital multi-channel broadcasting based on a communication satellite has been already practically used. Referring to FIG. 9, the configuration of a digital broadcasting system of the conventional art will be described. FIG. 9 is a diagram showing the configuration of a digital broadcasting system 151 of the conventional art.
The digital broadcasting system 151 comprises a transmission system 102 which produces and transmits a transport stream, and a reception system 153 which receives the transport stream and outputs it to a display device 4 or a recording device 5. A satellite transponder 6 is a device which relays a radio wave, and mounted in a communication satellite 7.
Next, the configuration of the transmission system 102 of the conventional art will be described.
The transmission system 102 has a program selection relating data producing section 109, a packet multiplexing section 10, etc.
A coding section 8 has an MPEG2 video encoder which performs digital video compression coding, and an MPEG2 audio encoder which performs digital audio compression coding. Namely, the coding section 8 produces a packet stream consisting of packets having contents of a program, and outputs the packet stream to the packet multiplexing section 10.
The program selection relating data producing section 109 produces a packet stream consisting of a packet having program specific information (PSI), and a packet having service information (SI), and outputs the packet stream to the packet multiplexing section 10. The program specific information is configured by an NIT (Network Information Table), a PAT (Program Association Table), a PMT (Program Map Table), and the like, and the service information is configured by an SDT (Service Description Table), and EIT (Event Information Table), and the like. As described above, the program selection relating data producing section 109 produces a packet stream consisting of packets having contents of program selection relating data, and outputs the packet stream to the packet multiplexing section 10.
The packet multiplexing section 10 time division multiplexes the packet stream consisting of packets having contents of a program and supplied from the coding section 8, and that consisting of packets having contents of program selection relating data and supplied from the program selection relating data producing section 109, into one transport stream.
A channel coding section 11 performs addition of an error correcting code, an interleave process, and addition of a synchronizing signal (framing) on the transport stream supplied from the packet multiplexing section 10. The interleave process is performed by changing the order of data transmission in the order of byte, in order to enhance the error correcting ability. The addition of a synchronizing signal is performed so as to provide a synchronization byte with periodicity, thereby enabling synchronization in the reception system 153 to be easily attained.
A modulating section 12 applies a modulation process due to quadrature phase shift keying (QPSK) modulation or the like, on a signal supplied from the channel coding section 11. A radio wave 147 is a transmitted radio wave.
Next, the configuration of the reception system 153 of the conventional art will be described.
The reception system 153 has a packet demultiplexing section 166, a stream sending section 172, etc.
A receiving section 13 receives a radio wave 148 which is transmitted from the satellite transponder 6.
A demodulating section 14 has a detector circuit which demodulates the radio wave 148.
A channel decoding section 15 restores the transport stream in which the order of data transmission has been changed in the order of byte by the interleave process in the channel coding section 11, to the original transport stream, and performs error correction on the restored transport stream.
The packet demultiplexing section 166 has a packet identifier (PID) filter 169, etc. That is, a PID identifies a corresponding packet.
A PID designating section 178 can designate the PID of a packet having contents of a record program, to a record PID storing device 18. Also, the PID designating section 178 can designate an NIT PID (i.e., the PID of an NIT packet), to an NIT PID storing device 23. As described later, a record program is a program which is selected by external instructions.
The record PID storing device 18 receives a signal from the PID designating section 178, and stores the PID of a packet having contents of a record program.
The PID filter 169 extracts a packet having contents of a record program which is indicated by the record PID storing device 18, a packet having program specific information, and a packet having service information, from a transport stream which is output from the channel decoding section 15. However, the PID filter 169 has a function of allowing the packet having program specific information, and the packet having service information to pass therethrough without applying any process on the packets.
An output switching circuit 20 switches over the transport stream which is supplied from the PID filter 169, to an output for displaying or that for recording.
An SIT (Selection Information Table) producing section 21 produces an SIT packet from the service information which is output from the PID filter 169.
The stream sending section 172 has an SIT packet replacing device 176, etc.
The NIT PID storing device 23 receives a signal from the PID designating section 178, and stores the NIT PID. As described later, an NIT packet is used as a replace packet which is replaceable with an SIT packet.
An SIT storing device 25 stores the SIT packet which is produced by the SIT producing section 21.
The SIT packet replacing device 176 can replace an NIT packet which is indicated by the NIT PID storing device 23, with the SIT packet which is output from the SIT storing device 25.
A packet modifying section 29 can receive a signal which is output from the SIT packet replacing device 176, and modify a packet to produce a record transport stream. The produced transport stream is output to the recording device 5.
A decoding section 27 has an MPEG2 video decoder which expands a compression coded video signal supplied from the output switching circuit 20, and an MPEG2 audio decoder which expands a compression coded audio signal supplied from the output switching circuit 20. Namely, the decoding section 27 expands a display transport stream which has compression coded contents of a program, and outputs the expanded stream to the display device 4. A display transport stream consists of transport packets containing video and audio contents.
The display device 4 has a cathode ray tube which reproduces a video signal supplied from the decoding section 27, and a loudspeaker which reproduces an audio signal supplied from the decoding section 27.
The recording device 5 records a record transport stream which is output from the SIT packet replacing device 176.
The operation of the thus configured digital broadcasting system 151 of the conventional art will be described with reference to FIG. 10 also. FIG. 10 is a conceptual diagram illustrating steps of producing a record transport stream 196 from a transport stream 144 which is broadcast.
Hereinafter, the operation of the transmission system 102 of the conventional art will be described in detail.
The coding section 8 produces a packet stream consisting of packets 130 and 138 having contents of a program A, and a packet stream consisting of packets 131, 135, and 139 having contents of a program B, and outputs the packet streams to the packet multiplexing section 10.
The program selection relating data producing section 109 produces a packet stream consisting of a PAT packet 132, a PMT packet 137, an NIT packet 134, an SDT packet 136, and an EIT packet 133, and outputs the packet stream to the packet multiplexing section 10.
The packet multiplexing section 10 multiplexes the packet stream supplied from the coding section 8, and the packet stream supplied from the program selection relating data producing section 109, to produce the transport stream 144. The transport stream 144 is then output to the channel coding section 11.
The channel coding section 11 performs addition of an error correcting code, an interleave process, and addition of a synchronizing signal on the transport stream supplied from the packet multiplexing section 10, and then outputs the transport stream to the modulating section 12.
The modulating section 12 applies a modulation process due to quadrature phase shift keying modulation, on the transport stream supplied from the channel coding section 11, and transmits the radio wave 147 from an antenna.
The satellite transponder 6 receives the radio wave 147, and then transmits the radio wave 148.
Next, the operation of the reception system 153 of the conventional art will be described in detail.
In response to external instructions, the reception system 153 selects the program A as a record program to start the receiving operation. Specifically, the reception system 153 receives the radio wave 148 from the satellite transponder 6, and refers information in the PAT in the transport stream of the radio wave, to detect the PMT. The PMT has the PIDs of packets for transferring a stream relating to the program A. The PID designating section 178 recognizes the PIDs of the packets 130 and 138 having contents of the record program, and performs a signal output to the record PID storing device 18. The PID designating section 178 recognizes the PID of the NIT packet 134, and performs also a signal output to the NIT PID storing device 23.
The record PID storing device 18 receives the signal from the PID designating section 178, and stores the PIDs of the packets 130 and 138 having contents of the program A. The NIT PID storing device 23 receives the signal from the PID designating section 178, and stores the PID of the NIT packet 134.
The receiving section 13 receives the radio wave 148, and then outputs the radio wave to the demodulating section 14.
The demodulating section 14 outputs the transport stream which is obtained by demodulating the radio wave 148 received by the receiving section 13, to the channel decoding section 15.
The channel decoding section 15 performs error correction on the transport stream supplied from the demodulating section 14, restores the transport stream 144, and outputs the restored transport stream to the PID filter 169.
As described above, the record PID storing device 18 stores the PIDs of the packets 130 and 138 having contents of the program A. Therefore, the record PID storing device 18 can indicate the packets 130 and 138 having contents of the program A, to the PID filter 169.
The PID filter 169 extracts the packets 130 and 138 having contents of the program A which are indicated by the record PID storing device 18, from the transport stream 144 output from the channel decoding section 15. Furthermore, the PID filter 169 extracts the PAT packet 132, the PMT packet 137, the NIT packet 134, the SDT packet 136, and the EIT packet 133 from the transport stream 144 output from the channel decoding section 15.
The PID filter 169 then outputs the SDT and the EIT to the SIT producing section 21. Furthermore, the PID filter 169 outputs to the output switching circuit 20 a transport stream 195 consisting of the packets 130 and 138 having contents of the program A, the PAT packet 132, the PMT packet 137, the NIT packet 134, the SDT packet 136, and the EIT packet 133.
The SIT producing section 21 comprehends the SDT and the EIT supplied from the PID filter 169, produces SIT packets 141 and. 142 from required service information, and outputs the SIT packets to the SIT storing device 25. The SIT storing device 25 outputs the SIT packets 141 and 142 supplied from the SIT producing section 21, to the SIT packet replacing device 176.
The output switching circuit 20 receives the transport stream 195 supplied from the PID filter 169.
If the output switching circuit 20 is set to the output for displaying, the output switching circuit 20 outputs the display transport stream to the decoding section 27. The display transport stream consists of the packets 130 and 138 having contents of the program A which have been filtered with reference to information in the PMT.
The decoding section 27 expands the display transport stream supplied from the output switching circuit 20, and outputs a signal of the expanded stream to the display device 4.
The display device 4 receives the signal output from the decoding section 27, and reproduces an image and a sound constituting the program A.
If the output switching circuit 20 is set to the output for recording, the output switching circuit 20 outputs the transport stream 195 to the SIT packet replacing device 176.
As described above., the NIT PID storing device 23 stores the PID of the NIT packet 134. Therefore, the NIT PID storing device 23 can indicate the NIT packet 134 to the SIT packet replacing device 176.
The SIT packet replacing device 176 receives the transport stream 195 supplied from the output switching circuit 20. The SIT packet replacing device 176 replaces the NIT packet 134 indicated by the NIT PID storing device 23, with the SIT packet 141 supplied from the SIT storing device 25.
In this way, the SIT packet replacing device 176 performs replacement of the NIT packet 134 with the SIT packet 141, and then outputs the signal to the packet modifying section 29.
The packet modifying section 29 receives the signal output from the SIT packet replacing device 176, and selects only specific information which correctly indicates the transport stream 195, whereby the PAT packet 132 is modified to a modified PAT packet 140, and the PMT packet 137 is modified to a modified PAT packet 143. The packet modifying section 29 discards the SDT packet 136 and the EIT packet 133.
In this way, the packet modifying section 29 produces a record transport stream 196, and outputs the stream to the recording device 5.
The recording device 5 receives the record transport stream 196 supplied from the packet modifying section 29, and records the transport stream.
In some cases, the transmission interval of an NIT packet is only about one time per 10 seconds depending on a broadcasting medium. This transmission interval is insufficient as the transmission interval for a replace packet which is replaceable with an SIT packet.
In the above, an NIT packet which is replaceable with the SIT packet 142 does not exist in the transport stream 195. Therefore, it is impossible to introduce the SIT packet 142 into the record transport stream 196. As a result, the SIT packet 142 is discarded.